At present, thin wires (bonding wires) having a diameter of approximately 20 to 50 μm are being mainly used as bonding wires for connection between the electrodes on a semiconductor element and external terminals. To bond a bonding wire, thermocompression bonding with the aid of ultrasound is commonly used, where a general-purpose bonding apparatus, a capillary jig into which a bonding wire is inserted for connection, or the like, is used. The tip of a bonding wire is heated and melted by arc heat input, thereby forming a ball by surface tension. Then the ball portion thus formed is compressively bonded onto an electrode of a semiconductor element that has been heated in a range of 150 to 300° C., and thereafter, the connected wire is directly bonded to the outer lead side by ultrasonic compression bonding.
Recently, the technologies relating to the structure, material, and connection for semiconductor device mounting have diversified rapidly. For example, regarding the mounting structure, in addition to current QFP (Quad Flat Packaging) using a lead frame, new types, such as BGA (Ball Grid Array) using a substrate, a polyimide tape, or the like, and CSP (Chip Scale Packaging) have been practically used. For this reason, a bonding wire having improvements in loop characteristic, bondability, mass production usability, and the like has been demanded.
Introduction of a fine pitch technique where a space between adjacent bonding wires becomes narrow has progressed. In response to the introduction of the fine pitch technique, thinning, improvement of strength, loop controllability, and bonding property have become requisite for bonding wires. Loop shape has become complex due to the density growth in the semiconductor packaging technologies. Loop height and wire length (span) of a bonding wire are barometers for classification of the loop shape. In most-recent semiconductor devices, contradictory loop shapes, such as a high loop and a low loop, and, a short span and a long span, are increasingly mixed within the interior of a single package. In order to realize such contradictory loop shapes with a bonding wire of one kind, strict designing of the material of the bonding wire is essential.
As the material for a bonding wire, high-purity 4N-group gold (purity>99.99% by mass) has been mainly used so far. To improve the properties, such as for strength enhancement, higher bonding, or the like, adjustment of a trace amount of an alloying element or elements has been performed. Recently, for the purpose of increasing the reliability of a bonded portion or the like, a 2N-purity gold alloy wire (purity>99% by mass) is in practical use, in which the concentration of an additive element is increased up to 1% by mass. Enhancement of strength, control of reliability, and the like can be achieved by adjusting the kind and concentration of the alloying element added to gold. On the other hand, bad influences, such as bondability degradation and increase in electrical resistance may occur due to such alloying, and therefore, it is difficult to comprehensively satisfy the various properties required for a bonding wire.
Moreover, because gold is expensive, another kind of metal whose material cost is lower is demanded, and thus, a copper-based bonding wire which is low in material cost and is excellent in electrical conductivity has been developed. With a bonding wire made of copper, however, there are problems in that bonding strength is lowered due to oxidation of the bonding wire surface, and wire surface corrosion or the like is likely to occur when encapsulated with a resin. These are the reasons why practical use of a copper bonding wire has not progressed.
All the bonding wires that have been used practically so far have the feature of a monolayer structure. Even if the material has been changed to gold, copper, or the like, these wires contain an alloying element distributed uniformly, and when they are seen in cross section, they are monolayer-structured. Although a thin native oxide film, an organic film for surface protection, or the like may be formed on the wire surface, these are limited within an extremely thin region (at a level of several atomic layers) of the outermost surface.
To meet the various needs for a bonding wire, a bonding wire having a multilayer structure where the bonding wire surface is coated with another metal has been proposed.
As a method of preventing surface oxidation of a copper bonding wire, Patent Literature 1 discloses a bonding wire where copper is covered with a noble metal or a corrosion-resistant metal, such as gold, silver, platinum, palladium, nickel, cobalt, chrome, or titanium. Moreover, from the standpoint of ball formability, prevention of deterioration of the plating solution or the like, Patent Literature 2 discloses a bonding wire structured to have a core material mainly composed of copper, a dissimilar metal layer formed on the core material and made of a metal other than copper, and a coating layer formed on the dissimilar metal layer and made of an oxidation-resistant metal having a higher melting point than copper. Patent Literature 3 discloses a bonding wire comprising a core material mainly composed of copper, and a skin layer formed on the core material and containing copper and a metal whose component or composition or both is/are different from the core material, where the skin layer is a thin film with a thickness of 0.001 to 0.02 μm.
Moreover, many multilayer structures have been proposed for a gold bonding wire also. For example, Patent Literature 4 discloses a bonding wire comprising a core material composed of high-purity Au or Au alloy, and a coating material coated on the outer surface of the core material and composed of high-purity Pd or Pd alloy. Patent Literature 5 discloses a bonding wire comprising a core material composed of high-purity Au or Au alloy, and a coating material coated on the outer surface of the core material and composed of high-purity Pt or Pt alloy. Patent Literature 6 discloses a bonding wire comprising a core material composed of high-purity Au or Au alloy, and a coating material coated on the outer surface of the core material and composed of high-purity Ag or Ag alloy.
These multilayer-structured bonding wires for a semiconductor device have not been used in practice so far although great expectation for practical use has been held. Surface reforming, high value-addition, and the like are expected by employing the multilayer structure. On the other hand, productivity and quality in wire production, yield and performance stability in the bonding process, long-term reliability during the use of a semiconductor device, and the like must be satisfied comprehensively.
Regarding the properties of a wire used for mass production, it is desired to cope with the most advanced high-density mounting, such as fine pitch bonding, stacked chip bonding, and the like by totally satisfying such required properties as stable loop control and improved bondability in the bonding process, wire deformation suppression in the resin encapsulation process, long-term reliability of a bonded portion, and so on.    Patent Literature 1: Unexamined Japanese Patent Publication No. S62-97360    Patent Literature 2: Unexamined Japanese Patent Publication No. 2004-64033    Patent Literature 3: Unexamined Japanese Patent Publication No. 2007-12776    Patent Literature 4: Unexamined Japanese Patent Publication No. H4-79236    Patent Literature 5: Unexamined Japanese Patent Publication No. H4-79240    Patent Literature 6 Unexamined Japanese Patent Publication No. H4-79242